1 Algae are tiny plant-like living things. Some algae live on land. They grow with lichens on rocks. Most algae live in water. At one time, algae were thought to be plants. But they are different from plants. Algae don't have roots, stems, or true leaves. Algae do have chlorophyll. Like plants, they use sunlight to make their own food. In the process, they make oxygen.2 Did you know that most of our oxygen is made by algae? How can this be? You might ask what about trees and other land plants? But remember that oceans cover about 71% of the Earth. Land only makes up about 29% of Earth's surface. Also remember that there are large chunks of land that don't have many plants at all. Antarctica and many large deserts have few plants growing there. Experts say that about 70 to 80 percent of our oxygen comes from algae! Some people think that algae are the most important living things on Earth!